Apparatus for use in testing an internal combustion engine ignition system

ABSTRACT

An apparatus for testing an i.c.e. ignition system of the kind including an ignition coil and a semi-conductor switch means controlling current interruption in the coil primary, includes a voltage comparator connected to compare the voltage across the switch means with a predetermined voltage. The comparator output starts a monostable timer circuit, the output of which is gated with the output of the comparator to produce an output signal only if the voltage across the switch means falls below the predetermined voltage within a predetermined time duration.

This invention relates to an apparatus for use in testing an internalcombustion engine ignition system, particularly a system utilizing anignition coil and electronic switch means for periodically connectingthe primary winding of said ignition coil across a supply and forperiodically interrupting the coil primary current.

With such a system it is difficult to establish in a test apparatuswhether or not the switch means is operating satisfactorily so as toproduce a spark. It is an object of the invention to provide apparatusfor testing electronic ignition systems in which this difficulty isovercome.

Apparatus in accordance with the invention comprises a voltagecomparator means for connection to the ignition system for detecting thevoltage across said switch means and indicating when such voltage is inexcess of a predetermined value which is an order of magnitude higherthan the system voltage and timing means associated with said comparatormeans and producing an output indicating a satisfactory interruption ofcoil current only when the output of the comparator means persists in astate indicating that said voltage has been above said predeterminedlevel for less than a predetermined duration.

Preferably said predetermined level is about 200 V for a 12 V system andsaid predetermined duration is about 20 μS.

The apparatus preferably also includes a lock-out circuit comprising afurther timing means connected to block the input to said first timingmeans for a second predetermined time (for example about 1 mS) aftersaid first timing means has produced said output.

An example of the invention is shown in the accompanying drawings inwhich FIG. 1 is a schematic circuit diagram of the test apparatus and

FIG. 2 is a graphical representation of waveforms at various points inFIG. 1.

The ignition system to be tested is shown at the lefthand end of FIG. 1.The system includes an ignition coil 10 having a primary winding 10a anda secondary winding 10b joined at one end. This common end of winding10a and 10b is connected by a ballast resistor 11 to the positive poleof a battery 13. An electronic switch 12, including an output transistor12a connects the other end of the primary winding 10a to earth. Theother end of the secondary winding 10b is connected via the usualdistributor 14 to the spark plugs 15.

The test apparatus derives its power from the battery 13 via a voltageregulator 16. A reference voltage generator 17 is also connected to thebattery 13 and provides a reference voltage which is applied to theinverting input of a voltage comparator 18. The non-inverting input ofcomparator 18 is connected to a point on a voltage dividing network 19,20 between said other end of the primary winding 10a of the coil 10 andearth and chosen so that when the voltage at said other end of theprimary winding 10a is equal to 200 V the voltages at the inverting andnon-inverting inputs of the comparator 18 are equal. Thus the output ofcomparator 18 is high whenever the voltage at said other end of theprimary winding is above 200 V.

The output of the comparator 18 is connected to one input of a NAND gate21, the output of which is connected to a monostable timer circuitcomprising capacitors 22, 23 resistors 24, 25, a NAND gate 26 and aninverter 27. The resistors 24, 25 connect the two inputs of the NANDgate 26 to the +ve rail, capacitor 22 connects the output of NAND gate21 to one input of NAND gate 26, capacitor 23 connects the output of theinverter 27 to the other input of NAND gate 26 and the output of NANDgate 26 is connected to the input of inverter 27.

The capacitor 22 and the resistor 24 acts to differentiate the output ofgate 21 so that this monostable timer circuit is triggered by a negativegoing change in the output of the NAND gate 21. The capacitor 23 andresistor 25 causes the output of NAND gate 26 to remain high for about20 μS following triggering.

A further NAND gate 30 has its two inputs connected to the outputs ofNAND gates 21 to 26 respectively, so that its output goes low only whenboth NAND gates 21 and 26 are producing high outputs. An inverter 32connects the output of gate 30 to the input of a further monostabletimer circuit comprising two capacitors 33, 34, two resistors 35, 36 andNAND gate 37 and an inverter 38, arranged to be triggered on by anegative going edge at the output of inverter 32 and to remain on forabout 1 mS.

The circuit has two output terminals, namely an output terminal 39connected via the capacitor 33 to the output of inverter 32, and anoutput terminal 40 connected to the output of gate 37. The output ofinverter 38 is connected to the other input of gate 21.

Trace A in FIG. 2 shows at its left hand end a voltage waveformassociated with a satisfactory interruption of coil primary current. Itwill be noted that the voltage rises very rapidly to a peak well inexcess of 200 V, but falls very rapidly (usually about 10 μS later) backbelow 200 V. The waveform may subsequently include peaks caused byresonance, interference etc which exceed 200 V, but if these occur after20 μS have elapsed they can be ignored. Trace B shows the output of thecomparator 18 and trace C the inverted version of this which appears atthe output of gate 21 whenever the output of inverter 38 is high (whichis its normal state). Trace D shows the 20 μs pulse generated by thetimer 22 to 27 following initial detection of the voltage exceeding 200V. The output of gate 30 (trace E) goes low only if this 20 μS pulsecontinues after the output of gate 21 has gone high again indicatingthat the voltage was above 200 V for less than 20 μS. If the output ofgate 30 does go low, the timer 33 to 38 is triggered when this outputgoes high again, thereby causing the output of inverter 38 to go low for1 mS and blocking gate 21 for this period.

The right hand end of FIG. 2 shows a situation in which, for variousreasons, the voltage remains above 200 V for more than 20 μS, indicatinga fault condition (for example electronic switch 12 turning of tooslowly)

It will be noted that trace E shows no negative going pulse so that thetimer circuit 33 to 38 is not triggered.

The circuit described is used in a test apparatus as disclosed inco-pending U.S. application Ser. No. 151,929 of even date based on U.K.Application No. 7,918,389. The pulses from terminal 39 are combined withpulses from another part of the apparatus indicating that current flowin the resistor 11 has restarted within a set time of the spike detectedby the circuit described herein, the combination of this spike with suchcurrent restarting indication a satisfactory operation. The pulseproduced at each satisfactory operation is passed to a processingcircuit including the missing pulse detector of co-pending U.S.application Ser. No. 152,127 of even date based on U.K. Application No.7,918,385 and the recognition circuit of co-pending U.S. applicationSer. No. 152,121 of even date based on U.K. Application No. 7,918,388.

I claim:
 1. Apparatus for use in testing an internal combustion engineignition system of the type utilizing an ignition coil and electronicswitch means for periodically connecting the primary winding of theignition coil across a supply and for periodically interrupting the coilprimary current to produce sparks, the apparatus comprising a voltagecomparator means for connection to the ignition system for detecting thevoltage across said switch means and indicating when such voltage is inexcess of a predetermined value which is an order of magnitude higherthan the system voltage and timing means associated with said comparatormeans and producing an output indicating a satisfactory interruption ofcoil current only when the output of the comparator means persists in astate indicating that said voltage has been above said predeterminedlevel for less than a predetermined duration.
 2. Apparatus as claimed inclaim 1 in which said predetermined voltage is about 200 V for a 12 Vsystem.
 3. Apparatus as claimed in claim 1 or claim 2 in which saidpredetermined duration is about 20 μs.
 4. Apparatus as claimed in claim1 further comprising a lock-out circuit comprising further timing meansconnected to block the input to said first timing means for a secondpredetermined time duration after said first timing means has producedsaid output.